Solid neon as a noise-resilient host for electron qubits above 100 mK

Abstract

Solid neon can be used as a solid host for single-electron qubits, and at temperatures of around 10 mK, electron-on-solid-neon charge qubits exhibit long coherence times and high operation fidelities. However, systematic characterization of the noise features of such systems is needed for the development of scalable quantum information architectures. Here, we show that solid neon can be used as a noise-resilient host for electron qubits above 100 mK. We examine the resilience of solid neon against charge and thermal noise when electron-on-solid-neon charge qubits are operated away from the charge-insensitive sweet spot and at elevated temperatures. We show that the extracted high-frequency charge noise density of electron-on-solid-neon qubits, projected as voltage fluctuations on nearby electrodes, is between $10^{-4}$ and $10^{-6}~\mathrm{μV^2/Hz}$ at 0.01 to 1 MHz, which is comparable with common semiconductor hosts. We also show that the electron-on-solid-neon charge qubits operating around 5 GHz frequencies can maintain echo coherence times of over 1 $μ$s at temperatures up to 400 mK.

Figures (5)

• Figure 1: eNe charge qubit coupled to a TiN high-impedance superconducting resonator. a, Illustration of the high impedance TiN superconducting resonator with two identical electron traps, microwave (MW) input and output couplers, and direct current (DC) gates. The metal plane between the resonator pins is connected to the ground plane via aluminium wire bonds. b, False-colour scanning electron micrograph image of the electron trapping area on the right side of the resonator with trap gates (RTG). Resonator bias voltage $V_{\text{res}}$ is applied symmetrically on the two pins of the resonator via the resonator gates in a. c, Cross-section schematic of the electron trapping area following the white dashed line in b, where the charge state of eNe qubit is coupled with the MW electric field $\vec{E}$ within the resonator.
• Figure 2: Spectroscopic and coherence properties of an eNe charge qubit Q1. a, Normalized microwave transmission amplitude ($A/A_0$) centred around the resonator frequency versus the relative resonator bias voltage $\Delta V_{\text{res}}$ as described in b. Two avoided crossings appear when the eNe qubit comes into resonance with the resonator. b, Two-tone qubit spectroscopy measurement displaying the transmission phase response at the resonator frequency $\omega_\text{r}$ versus $\Delta V_{\text{res}}$, while weak drive tones concurrently sent in at a frequency of $f_{\text{drive}}$. c, Ramsey fringes at the charge sweet spot, marked with the red arrow in b, with fitted total dephasing (Ramsey) time $T_2^*$ of 8.2 µ s. $P_\text{e}$ is the qubit's excited-state population. d, Relaxation and Hahn echo measurements showing total decoherence time with a Hahn echo $T_2^{\text{echo}}$ = 21.6 µ s and relaxation time $T_1$ = 11.6 µ s at the charge sweet spot.
• Figure 3: Decoherence of eNe qubit Q1. a, Extracted total decoherence rate under CPMG sequences $1/T_{2}^{\rm{CPMG}}$ from experiments with Qubit 1 (Q1) biased at various frequency lever-arm $|\partial f_\mathrm{q}/\partial V_\mathrm{res}|$, and with refocusing pulse numbers $N =$ 0, 1, 2, 4, 6, 8, 12, and 16, denoted by different colours. With the increase of $N$, the $1/T_{2}^{\rm{CPMG}}$ of the qubit biased near the charge sweet spot approaches the limit of $2T_1$. b, The pure dephasing time $T_{\phi}$ increases as a function of $N$ when the qubit biased away from charge sweet spot, with a power-law fit of $T_{\phi} \propto N^{0.6(1)}$. c, Repeated Ramsey fringes measured near qubit sweet spot for 128 iterations, with each record taking 33 s. d, Detuning $\Delta_{dq}$ between drive tone and qubit frequency during the Ramsey measurements, revealing stochastic frequency shifts. All error bars represent the one standard error of extracted parameters (Methods).
• Figure 4: Noise spectroscopy of eNe qubit Q1. a, Data between 0.01 $\sim$ 1 MHz: Total longitudinal noise density (coloured dots) derived from dynamical decoupling data at different qubit bias points. Data between $10^{-3}$ to $10^{-1}$ Hz: Extracted total longitudinal noise (blue dots) from long-term Ramsey measurements when biased at charge sweet spot. Data near 5.0 GHz: Transverse noise of the eNe qubit (green diamonds). $S(f)$ represents the noise spectral density with $f$ denoting the frequency. b, Voltage noise density $S_\mathrm{v} (f)$ between 0.01 and 1 MHz, projected on the resonator electrode. c, Zoom-in of the transverse noise in a between 5.0 and 5.6 GHz. Notice that the x-axis is linear with the unit of GHz. Gray dashed lines: Power-law fits of frequency-dependent noise.
• Figure 5: Temperature-dependent coherence of eNe qubit Q1 at charge sweet spot. a, Relaxation time $T_1$ (blue dots, data) versus mixing chamber (MXC) temperature. The solid curve represents the predicted $T_1(T) = T_1(T=0)\cdot\text{tanh} (\hbar \omega_{\rm{q}}/2k_BT)$ where we use the measured value at 10 mK for $T_1(T=0)$, and the MXC temperature as $T$. Extracted electron temperature (gray diamonds, data) versus MXC temperature based on the measured thermal population in Supplementary Information Section 3. b, Decoherence time, $T_2^*$ (orange squares) and $T_2^{\rm{echo}}$ (green triangles) versus the MXC temperature, with power-law fitting (dashed curves). Gray dots and curves show $2T_1$ for comparison. c, Extracted pure dephasing time $T_{\phi}$ as a function of the MXC temperature. Red dashed curve: Parameter-free calculation of the resonator-induced dephasing based on Q1 properties. All error bars represent the one standard error of extracted parameters (Methods).